This invention relates to an amplifier for an electric guitar and, in particular, to a simulated tone stack in such an amplifier.
An electric guitar can have either a solid body or a hollow body, the latter generally being referred to as an electro-acoustic guitar. Both types of guitars include a transducer for converting the vibration of the strings to an electrical signal. The transducer in an electro-acoustic guitar is typically a piezoelectric element coupled to the strings at the junction of the strings with the body of the guitar. In a guitar with a solid body, the transducer is typically magnetic and is located near the junction of the strings with the body of the guitar. The invention can be used with either type of guitar, although the following description is primarily in terms of a guitar having a solid body.
An amplifier for an electric guitar has low fidelity and the low fidelity contributes to the "voice" of the guitar to such an extent that the guitar and the amplifier together are the instrument, not the guitar alone. The unique sound or voice associated with particular electric guitars comes from several sources including a tone control circuit, overdriven amplifier stages, other voice equalization stages, and the audio characteristics of the speaker element and the enclosure. Many patents, such as U.S. Pat. No. 5,789,689 (Doidic et al.) and U.S. Pat. No. 5,977,474 (O'Brien), are concerned with simulating an overdriven tube type amplifier in a transistorized amplifier. This invention concerns a tone control circuit, not an imitation tube circuit or circuits for producing various effects.
A tone control circuit, sometimes referred to as a tone stack, for a guitar amplifier is typically a variation on a circuit such as shown in FIGS. 1-4. The O'Brien patent discloses one such tone control circuit. Slight rewiring and, more often, changes in component values produce distinct voices for each circuit. There are many variations of the circuit in existence. If a performer wants a particular sound, he must obtain the amplifier that has the tone stack for that sound. Amplifiers with more than one tone stack are not made, partly because most manufacturers are striving for a unique sound. On the other hand, providing several complete sound systems for a concert with several performers is costly and inconvenient.
Simulating several tone stacks with a single programmable amplifier is difficult for several reasons. A first reason is that portions of the tone stack interact; that is, the bass, mid-range, and treble controls are not isolated elements. A second reason is the number of combinations obtainable from relatively few elements. Specifically, reproducing the response curves for each tone control circuit at each possible setting of bass, mid-range, and treble involves millions of combinations of parameters. A third reason is that each tone stack has its own set of parameters, further increasing the amount of data.
It is known in the filter art to create a desired frequency response by coupling a signal directly to the non-inverting input of an amplifier and coupling a filtered signal to the inverting input of the amplifier; see Electronic Filter Design Handbook by Williams and Taylor, Third Edition, McGraw-Hill, Inc., 1995, pages 6.30-6.31. As disclosed in the text, a filtered signal is inverted and subtracted from the unfiltered signal to produce the desired response.
Interpolation, in mathematics and as implemented in a computer, is known in the art. In its simplest, one dimensional form, interpolation is approximating an unknown value between two known values. For example, children are taught to estimate by comparing results. If 5.sup.2 =25 and 6.sup.2 =36, then one might reasonably estimate that (5.5).sup.2.apprxeq.30.5, which is half-way between 25 and 36 and is not a bad estimate for the actual value (30.25). A far more elegant treatment of the subject is provided in .sctn.3.6 "Interpolation in Two or More Dimensions" Numerical Recipes in C, Cambridge University Press 1992.
In view of the foregoing, it is therefore an object of the invention to provide a circuit for simulating many tone stacks in a single amplifier for an electric guitar.
Another object of the invention is to provide a circuit that can simulate each of a plurality of tone stacks and that is compatible with circuits for producing linear and non-linear effects in an audio amplifier.
A further object of the invention is to provide a programmable tone control circuit.